What should I do to stop blender freezing on start-up, when its okay starting from a double click .blend file
startup file can be messed up.

Open with a file that's ok, reload factory settings and save starup file.

If preferences (addons or anything) are messed up, do the same but open prefs and click save user settings.

If something's wrong in program data or manually added scripts/addons, explore C:\Users\ [userName] \AppData\Roaming\Blender Foundation\Blender and delete everything (or move).

After a crash, blender files could be corrupted. You can uninstall-reinstall blender to fix them.

If nothing helps, you can check your disks (all where windows, blender and .blend files are) from explorer RMB on the disk you want to check, properties, tools tab, check now. In the pop up, check automatic repair, uncheck search and fix defective sectors (or something like that, translating). This won't last long.

If this fails at fixing, try to check fix defective sectors but it'll take more time and for windows disk, you'll need to restart (and wait).